Variable inductor



Oct. 27, 1959 L. A. GLYNN VARIABLE INDUCTOR Filed April 9, 1956 United States Patent VARIABLE INDUCTOR Leon A. Glynn, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application April 9, 1956, Serial No. 577,141

2 Claims. (Cl. 336-439) This invention relates to variable inductor devices and more particularly to contact roller arrangements.

Prior art devices for variable inductors used in tank circuits in the high power class have usually comprised a rotatably-mounted coil of self-supporting wire and two contact rollers, one used for circuit connection and the other to short circuit a portion of the coil in conjunction with the circuit roller. In some circuit arrangements the short-circuit current becomes appreciably higher than the tank currents. Under this condition contactors handling both tank currents and shorted turns current become overloaded, heat, arc, and generally fail. This failure occurs in inductors used in radio frequency transmitters of as loW a power as one kilowatt.

Accordingly, it is an object of this invention to provide a contact roller arrangement which greatly reduces the burning of contacts due to heating and arcing during use.

It is a further object of this invention to provide a roller contact arrangement which separates the shortedturns current from the tank current and so provides a more reliable contact roller system.

It is a further object of this invention to minimize the effect of the short circuit on the Q, or quality factor, of the coil.

It is a feature of this invention that a plurality of rollers are used, each for separate purposes, with insulation provided in appropriate places to prevent mutually undesired currents.

It is a further feature of the invention that the rollers are mounted so as to have equalized contact pressures.

Further objects, features, and advantages of the invention will become apparent from the specification and claims when read in conjunction with the drawing, in which:

Figure 1 shows a section view of a rotary variable inductor,

Figure 2 shows a perspective view of the details of the roller contact system, and

Figure 3 shows an equivalent circuit diagram for the inductor.

In Figure l a self-supporting coil is mounted between insulating end plates 11 and 12. Coil 10 is shown having rectangular cross section wire with the Wide dimension of the wire in the radial direction of the coil. Other forms of wire may be used as long as they are self-supporting, and do not deflect unduly because of the spring pressure of the roller contacts. The wire will preferably be copper with a silver plating for improvement of high frequency conductivity and corrosion re sistance. The coil may have longitudinal supporting strips in some applications, provided the strips do not interfere with the contact rollers.

End plates 11 and 12 are insulators and may be ceramic or plastic as desired. End plates 11 and 12 are mounted on a shaft 13 which is rotatably mounted in bearings 14. Bearings 14 may be sleeves such as solid or sintered bronze operating on the steel shaft 13. Roller or ball bearings may be used as desired, such as 2,910,661 Patented Oct. 27, 1959 P ice where high radial loads are involved. Necessarily, bearings 14 will also provide for control of shaft end-play, necessary for proper contact pressures of contacts 15 and 16. The thrust from the force required to move the roller assembly must also be accommodated. Contacts 15 and 16 run on slip rings 17 and 18, respectively, which are connected to the ends of the coil 10. Slip rings 17 and 18 may be of copper with silver plating or may have a plating such as chromium or rhodium for improvement of wear resistance against the rubbing Wear of the brushes. Brushes 15 and 16 may be made of springy beryllium copper alloy with a plating such as silver to enhance the high frequency conductivity and to improve corrosion resistance. Brushes 15 and 16 are mounted on insulated posts or terminals 20 and 21, respectively, which are available for connection with the rest of a circuit, such as a tuned circuit.

A knob 22 is shown mounted on shaft 13 for illustration of the rotation of the coil 10. Rotary positioning means, manually or automatically operated, or coupling onto other rotated shafts, may be used to rotate coil 10.

Contacting the central portion of coil 10 is a roller assembly 25. This roller assembly consists of a pair of' axles, each having a pair of rollers. A bar 26 carries these four rollers by their axles. This bar 26 is pivotally mounted on a post 27 having the shape of an inverted L. The transverse portion of post 27 carries bar 26 and has retainers 28 on either side to keep bar 26 in place. Post 27 is mounted in a block 30. This block closely fits and is carried by channel 29. Bar 26, post 27, block 30, and channel 29 each may be metallic, such as brass or copper or possibly non-metallic if the occasion calls for it, but have in any case a plating of silver for improvement of the conductivity at high frequencies. Bar 26 is freely movable on post 27, pivoting about the short leg of post 27. Block 30 is a close fit in channel 29 but is readily moved along, parallel to the axis of the inductor 10. Block 30 has suflicient clearance to move along freely, relative to the strength of turns of the coil 10 to provide the advancing force, yet has sufficient contact pressure against channel 29 to make a good contact.

Channel 29 is mounted by extensions either of the channel itself or by separable extensions 31 which are mounted on the channel. Extensions 31 are carried on bolts 32 and 33. These bolts have nuts 34 on top and springs 35 below the extensions 31. Springs 35 are under compression to the extent needed to provide the desired contact pressure of contact roller assembly 25 against coil 10. Nuts 34 prevent the assembly from coming apart, and also provide stops to prevent further deflection of the coil such as when the contact assembly is at the middle. A flexible lead 36 connects channel 29 electrically to the same terminal 21 as brush 16. The channel may have other terminals thereon in accord with the requirements of the circuit. A frame 37 is arranged in any desired manner to support the shaft 13 and the channel 29 in the cooperative relation shown.

Figure 2 shows a perspective View of the coil 10 and contact roller assembly 25. Here the supporting channel 29 can be seen supporting a block 30. Post 27 is barely visible but supports bar 26 and positions it through the use of retaining rings 28. Bar 26 has notches therein spaced parallel and substantially parallel also to the axis of the coil 10. In these notches are supported two axles 40 and 41. Axle 40 is metallic or ultimately has a conductive metal coating. This coating conducts high frequency currents to bar 26 from roller 44, and may be either silver, rhodium or other, as the use demands. Axle 41 is insulated by a plastic insert 42 such as nylon or Teflon, preferably Teflon. Nylon and Teflon are well-known plastics having high wear strength, good dielectric qualities and low chemical activity. Nylon is a plastic composed of polyamide resins, while Teflon is a solid polymer of tetrafluoroethylene.

On: axle 40 are mounted two rollers. One, 44, is conductive or has a conductive surface connected to axle 40; the other, 45, is of insulating material or has. at least an insulatingsurface. Roller 45 may be made of a solid plastic as noted above, and is preferably all Teflon. Roller 44 is designated the signal or load current roller. That is. to say, the load current, or the tank current where the inductance is in a resonant circuit tuned by the variable tap to coil 10, is carried by roller 44 from the, coil through axle 40, bar 26, post 27, to channel 29 and. on to the external circuit. Roller 45 does not carry any current by virtue of being an insulator. Roller 45 provides a balancing force to that of roller 44 so that axle 40 is kept aligned and the rollers in contact with thecoil 1t). Axle 40 has somewhat of a pivotal relation relative. to bar 26 so that the pressure is equalized between the two rollers 44 and 45.

Axle 41 carries two conductive rollers 46 and 47. Rollers 46 and 47 may also be made of insulating material with a. conductive coating. Axle 41 is contacted electrically by both rollers for conduction of currents.

between the turns contacted by the two rollers. Rollers 46 and 47 are designated the shorted-turns current carrying rollers, and contact the coil in the inactive portion. These rollers are intended to carry the current between the coil turns which they contact and short together. Axle 41 has somewhat of a pivotal relation with respect to bar 26 also. Bar 26, as stated before, is pivotally mounted on the short leg of post 27. Thus, the pressure of springs 35, seen in Figure 1, urges channel 29 in radially, toward the coil. This pressure is transmitted to block 28 from channel 29. The pressure on block 28 is transmitted through post 27, bar 26, axles 40 and 41, to thefour rollers which contact coil 10.

Thus it can bev seen why coil 10 must have considerablev rigidity. Since the coils requiring this type of shorting treatment generally require a large wire size because of thehigh radio-frequency currents involved, there is little difiiculty in making the coil strong enoughalso for the contact pressure of the roller assembly 25. The shape of the grooves of the contact rollers is made to correspond with the shape of the wire used.

Figure '3 shows the effective circuit diagram of the assembly shown in- Figures 1 and 2. Terminal 2! has the same number as the corresponding terminal in Figure l; likewise, terminal 21. Roller 44 is the variable tap of. the inductance; the current conducting path from roller 44 through lead 36 is connected to terminal 21. Brushes 16 and make the contacts to the ends of the coil. Shorting rollers 46 and 47 and axle 41 make the similarly numbered shorting loop seen in Figure 3.

In operation, the coil is rotated by shaft 13. Rollers 44,45, 46, and 47 roll freely and follow the helix of the coil. The rollers are moved axially along the inductance by rotation thereof. The contact rollers make a continuous contact between the coil and the frame as seen in Figure 3.

Thev active portion of the coil between terminals 20 and21 is the inductance available to the circuit external to these two terminals. The inductive coupling from the active portion of the coil into the inactive portion is reduced substantially to zero by the two shorted turns encompassed between rollers 46 and 47. In actual use there is little discernible difference between the end of the coil contacted by brush 16 being connected to terminal 21 or being left free Without any connection. When a load or tank current flows'through the active portion of the coil, a shorted-turns current is induced in the section of the coil between rollers 46 and 47. There is also a coupling to the inactive portion of the coil between contact roller 44 and the end. The coupling into the varying inactive portion is reduced, however, to substantially zero by the counter electromagnetic flux generated in the shorted turns. One or more than two turns are similarly effective. A one turn short, however, causes unduly high currents, while more than two turns does not provide a great enough short-circuit current to buck out completely coupling to the unused part of the coil. This, of course, is a function of the operating frequency and is proportionate to the coil characteristics.

As a result of this invention, a variable inductance is made available with no coupling to an inversely variable unused portion of the coil, which ordinarily produces undesirable effects such as loading, variation of Q, and undesirably high induced voltages where the turns ratio becomes great. These advantages are made available with a considerable reduction of contact heat. Further, by the shorting rollers being isolated from the load current roller and contacting the unused portion of the coil, the Q of the coil in circuit is not adversely aifected.

Although this invention has been described with respect to particular embodiments thereof, it is not to be so limited because changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claims.

I claim:

1. A variable inductance comprising a self-supporting helical conductor supported at the ends by rotatable means, a contact roller assembly comprising two pairs of rollers elastically urged against said helical conductor, means pivotally mounting each pair of rollers, movable means pivotally mounting said pivotal means, said movable means being movable substantially parallel to the axis of said helix, a fixed terminal, one roller of one pair of rollers being conductively connected to said fixed terminal through said parallelly movable means, the other roller of said one pair being insulating at its contact with said coil, the rollers of said other pair being conductively interconnected so as to short-circuit turns of the helix contacted, and insulation means between said other pair of rollers and said pivotal mounting means.

2. A variable inductor comprising a helical conductor, insulating means bounding said inductor at each end, rotatable means mounting said insulating means so that said helix may be rotated about its axis of formation, contact roller means for contacting a selected point on said hel x, said contact means comprising a first pair of rollers mounted on a first axle, a second pair of rollers mounted on a second axle, one of said first pair of rollers being conductive, the second of said first pair of rollers being insulating, said second pair of rollers and axle being conductive, pivotal mounting means carrying said first and second axles, insulating means supporting said second axle in non-conductive relation to said pivotal means, sliding contact means carrying said pivotal means, means mounting said sliding contact means adjacent said helix, said roller assembly being movable along the axis of said coil, and terminal means connected to said slideable mounting means whereby a conductive path from said conductive roller of said first pair of rollers to said terminal means is established from said point selected by rotation of said helix about its axis.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,443 Craymer Dec. 6, 1938' 2,178,221 Blancha Oct. 31, 1939 2,266,532 Braunschweig Dec. 16, 1941 2,480,340 Rose Aug. 30, 1949 2,503,582 Ginsburg Apr. 11, 1950 

